1. Field of the Invention
The present invention relates to a method of perch channel transmission and cell selection in a CDMA (Code Division Multiple Access) mobile communication system in which a plurality of base stations provided in respective cells transmit perch channels which are spread by using mutually different spread codes assigned to these base stations and each mobile station judges a located cell by receiving the perch channels.
2. Description of the Background Art
As an example of the conventional mobile communication system, there is a digital mobile communication system called PDC (Personal Digital Cellular) system which is currently providing a mobile communication service in Japan, in which the mobile station carries out the cell selection by receiving a perch channel that is transmitted from each base station continuously at a constant level. Then, the mobile station measures receiving levels of a plurality of perch channels that are constantly transmitted from a plurality of base stations in the surrounding, and judges that it is currently located in a cell from which the perch channel with the largest receiving level is transmitted.
On the other hand, in the CDMA mobile communication system, the system capacity is determined by an amount of interference power, and the system capacity is reduced when an amount of interference power is increased. In order to prevent an increase of an amount of interference power, there is a need to make the transmission power of a various type of radio channel as small as possible within a range in which the required receiving quality can be satisfied at a receiving side. In this regard, because of its use for the purpose of the cell selection by the mobile station, there is a need for the perch channel to be transmitted at a relatively large transmission power compared with the other radio channels so that the perch channel can be received even by a mobile station which is located in a neighboring cell.
Now, when the perch channel is transmitted continuously in the CDMA mobile communication system similarly as in the conventional PDC system, the interference power will be increased because of the relatively large transmission power of the perch channel, so that there is a problem that the system capacity will be reduced.